wined3d: Move float_32_to_16() to surface_base.c which is the only place where it's used, and because it needs port.h for isinf().

Flushing blocks any end of stream that may occur.

No need to expose this in the interface.

Previously the cursor was either an IBEAM, or a reversed arrow when over
the selectionbar.

When the cursor is moved over the selection bar, without holding any
mouse buttons down, the cursor would be repeatably set between the
normal cursor, set by DefWindowProc for the WM_SETCURSOR message, and
the reversed cursor, set by ME_MouseMove.